Abstract

A mineralogical, microstructural and geochemical approach was used to trace the postdepositional alteration of molluscs and to quantify the biogenic aragonite to calcite transformation in the Boggy Formation (mid-Desmoinesian) of Oklahoma. The results show that with progressive alteration, primary biogenic aragonite was gradually replaced by diagenetic low-Mg calcite. During the initial phase of alteration, the nacreous tablets of the shell layers fused, and with progressive postdepositional diagenesis, they were replaced by either fine- or coarsegrained calcite. During this diagenetic transformation, no significant changes in chemical concentrations were noted for Ba, Cu, Ni, Pb, and Zn; whereas Mg, Mn, and Fe increased by factors of about 14 to 25 relative to their depositional concentrations. Concurrently, Na, Sr, δ18O, and δ13C concentrations decreased by factors ranging from 2.5 to 10. Under similar diagenetic conditions, matrix samples were more altered than were contemporaneous fossils, and it is postulated that grain size/surface area was a major control on the degree of alteration of carbonate grains. The matrix of the Boggy Formation has retained sufficient geochemical information to suggest an original aragonitic limemud precursor of nonmolluscan affinity. With the geochemical methods and quantified trends, the least-altered mollusc samples of the Boggy Formation, consisting of only aragonite, can be identified.

Geochemical data of the most-altered samples suggest that the diagenetic fluids which affected Boggy sediments and fossils were similar in composition to meteoric water. Moreover, the dissolving aragonite, through diffusion-controlled dissolution, determined the elemental as well as isotopic chemistry of the diagenetic calcite in the altered shells. It is postulated that similar diagenetic waters and processes affected sedimentary units of the Breathitt (Pennsylvanian, Kentucky), Brush Creek (Pennsylvanian, Pennsylvania-Ohio), Lukow (Jurassic, Poland), Bear Paw-Lea Park (Cretaceous, western Canada), and Nugssuaq (Cretaceous, Greenland) Formations which contain well-preserved aragonitic molluscan faunas.

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